1. Field of the Invention
The invention relates generally to the field of fluid cooling of semiconductor components and in particular to a fluid-cooled power transistor arrangement.
2. Description of the Prior Art
In order to control electric devices and machines, semiconductor valves are used in great numbers. The type of valve to be used is determined, first of all, by the quantity of power to be controlled and, secondly, by the maximum operating frequency. Thyristors and triacs are used at network frequencies, i.e., on the order of 50 Hz, and permit power control to on the order of 10 megawatts. In many application cases, however, especially in the control of electric machines, higher switching frequencies up to nearly the megahertz range are required. For applications of this type, power transistors are used. In the frequency range around 10 kHz at powers on the order of between 10 and 100 kW, BIMOS power transistors and IGBT (Insulated Gate Bipolar Transistor) power transistors are used. Moving toward higher frequencies, but at lower powers, MOSFET power transistors are normally used.
Power semiconductor components must be cooled. In the active region of the semiconductor component, temperatures should not rise above relatively low temperature values. The waste heat passing through not only the semiconductor substrate, but also the electrode platings and the multi-layer supporting plates on which the semiconductor substrate is located must be eliminated. In power transistors of the type discussed above, the semiconductor substrate has, on at least one of its sides, a base metal plating which overlaps in planar fashion the entire active region of the substrate and, depending on the type, forms the collector or the drain electrode. The other electrodes of the transistor, i.e., the base electrode and emitter or the gate or source electrode, are accessible on the opposite flat side of the semiconductor substrate. In conventional power transistors, a fluid-cooled heat sink arrangement is attached to the flat base metal plating and dissipates the waste heat of the active region of the transistor through the semiconductor substrate and the base metal plating. Because the temperature in the active region must be maintained uniformly within the given limit values, it is vital that the heat sink arrangement be attached to the semiconductor substrate of the semiconductor element flatly and with uniform heat exchange characteristics. As a rule, a direct connection of the heat sink to the semiconductor substrate is not possible in view of the high voltages (1000 V and more) and high currents (e.g. 100 amperes), which means that the semiconductor substrate must be arranged on an insulating substrate via which, in previous power transistor arrangements, the waste heat has had to be dissipated from the semiconductor component into the cooling arrangement. It is therefore common to place the semiconductor component on a ceramic board which has copper plating on both sides and to solder the ceramic board by its side away from the semiconductor component onto a supporting plate of for example, steel. In turn, the steel plate is placed with an intermediate layer of a heat conducting paste onto the, for example, water-cooled cooling element. Suitable heat sink arrangements are known, for example, from EP-A-447 835. However, it has been shown that the switching capacity of power transistors often cannot be fully utilized or that malfunctions of power transistors occur if inhomogeneities, which can lead to local overheating of the semiconductor component and thus to the destruction of the transistor, remain in the solder layer connecting the ceramic board to the steel plate or in the heat conducting paste layer.
In order to improve the cooling effectiveness of power transistors, a method is known of providing the supply bands attached to the side away from the substrate with heat sinks, which strengthen the cooling of the active region of the semiconductor component (EP-A-252 429 and EP-A-449 435). In addition, a method is known (EP-A-260 370) of attaching a heat sink equipped with cooling ribs in a material-tight fashion to the flat side of the semiconductor component away from the active area and exposing the cooling ribs to a cooling air flow.
Finally, a method is known from DE-A-41 01 205 of arranging the boardlike semiconductor component of a power diode or a power thyristor in a coolant channel and contacting it on both sides by flexible contact brushes. Each contact brush consists of a multiplicity of wire pieces, individually parallel to one another, which are cooled by the cooling fluid flowing along the semiconductor component. However, the contact brushes do not permit any surface heat dissipation, as would be required for cooling the semiconductor element of a power transistor. In DE-A-41 01 205, it is suggested that the cooling fluid be water, air, oil or a cooling agent with hydrocarbon content.